This invention relates to anion-containing aqueous disperse systems containing amphoteric polyelectrolytes as coacervating agents and to a method for accomplishing coacervation of such systems.
It is well known that the particles of an aqueous polymer dispersion have a superficial electrostatic charge, which is particularly appreciable when the dispersion is prepared with the help of an ionic surface active agent. See, for example, Moillet et al., Surface Activity, Spon. Ltd., London, 1961 and Schildknecht, Polymer Processes, Interscience Pub., 640-8 (1956). Thus, the particles in many known aqueous dispersions of organic and inorganic materials have a surface charge which is positive or negative, depending in part on whether or not the dispersing agent used was cationic or anionic. Of particular interest in industry are the aqueous dispersions in which the dispersed particles carry an anionic charge. The sign of the charge, if not known, can be readily determined by adding a polycation. If precipitation occurs, the electrostatic charge on the particles is anionic.
It is also well known to dissolve certain polymers bearing anionic groups such as poly(acrylic acid), poly(methacrylic acid) and copolymers of such acids with other ethylenically unsaturated monomers wherein the acid constitutes above about 30 weight percent of the copolymer in water with or without the aid of added base such as sodium hydroxide or added organic solvent such as alcohol, dioxane, acetone and tetrahydrofuran. Other such water-soluble polymers include poly(sodium styrene sulfonate), polymers of sulfo esters of unsaturated carboxylic acids such as poly(sulfoethyl methacrylate), and poly(ethylene sulfonate). Further, it is well known to dissolve non-polymeric organic compounds bearing anionic groups, e.g., anionic surfactants such as ammonium oleate and sodium stearate, in aqueous media.
Such aqueous disperse systems, inclusive of dispersions and solutions, are frequently employed in the coating of a variety of substrates, spinning of fibers, impregnation of paper or other bibulous substrates, lamination of sheets, etc. Fluidity of such disperse systems which is conferred by the aqueous medium is generally desired during application. However, once the application is completed, it is desirable to separate the mobilizing medium, i.e., the aqueous phase, from the dispersed material. In most instances, the utility of the dispersed material depends upon its immobility, inertness, or non-redispersibility when deposited. Aqueous dispersions of normally solid, organic polymers so-called latexes, are most often utilized for these purposes. In other disperse systems, it is often desirable to flocculate or precipitate dispersed solids in order to merely collect dispersed solids and/or to remove such solids from the dispersing aqueous phase. Following precipitation, it is, in some systems, desirable to redisperse the precipitated material to its original disperse state, e.g., as a means to recover unused or excess materials and materials used in recycling operations.
Several methods are known and are widely used in industry for precipitating collodial dispersions and solutions and for coagulation of natural and synthetic colloidal systems. Among such methods are heating, cooling, the use of electrical current, violent agitation and the addition of chemicals. Among the latter, the precipitation of latexes, emulsions and other disperse systems by the addition of salts, especially salts of the alum type and/or the addition of acid to change the pH of the colloidal system are known. See, for example, Schildknecht, supra at page 640 and Jirgensons, Organic Colloidals, Elsevier Publishing Co., 223-239 (1958). See also U.S. Pat. No. 2,995,512, U.S. Pat. No. 3,528,928, U.S. Pat. No. 2,832,746 and U.S. Pat. No. 3,006,868.
The coacervation methods of the prior art have several disadvantages, particularly in certain specialized applications. For example, in the use of most chemical coagulation or gelation chemical methods of the prior art, coagulation or gelation occurs almost instantaneously after the addition of the chemical coagulating or gelling agent. In some prior art methods, gelation can be delayed for a period of time. See, for example, Madge, Latex Foam Rubber, Interscience Pub., 23-31 (1962). In either instance, an anionically-stabilized, aqueous disperse system compounded with the coagulating or gelling agent cannot be stored and subsequently used in stable, fluid form. Furthermore, in prior art gelation of latexes, the rate and uniformity of gel formation is not easily controlled once the gelling agent is added. In most prior art gelation, the fluidity and foaming of the wet froth of the latex is often impaired and even destroyed. In special reactive latexes such as described in U.S. Pat. No. 3,215,647 to Dunn, conventional coagulating or gelling agents are not effective to form firm wet gels which can be readily handled. In some systems wherein redispersibility is desired, the coagulation caused by prior art techniques is irreversible.
Since it is often necessary, and certainly beneficial, to effect controlled coacervation of aqueous disperse systems without adding chemical coacervating agent at the time coacervation is required, it would be highly desirable to provide stable, fluid, aqueous disperse systems wherein the disperse material contains anionic groups which disperse system can be stored for long periods of time, but which can be coacervated immediately with or without addition of chemical coacervating agent. The latter technique of coacervation is defined herein as latent coacervation.